Improve splat promotion to handle AVX types: v32i8 and v16i16. Also refactor the code and add a bunch of comments. The final shuffle emitted by handling 256-bit types is suitable for the VPERM shuffle instruction which is going to be introduced in a next commit (with a testcase which cover this commit)
Improve splat promotion to handle AVX types: v32i8 and v16i16. Also
refactor the code and add a bunch of comments. The final shuffle
emitted by handling 256-bit types is suitable for the VPERM shuffle
instruction which is going to be introduced in a next commit (with
a testcase which cover this commit)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@135661 91177308-0d34-0410-b5e6-96231b3b80d8

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